The present invention relates to a planar diffractive relay which may be mounted on the head or on a helmet to view a display, and more particularly, to view the display of a night vision device to provide night time viewing.
Night vision systems include objective lenses, image intensifier tubes and eyepieces usually assembled in a straight line. Many night vision goggles extend out in front of a viewer""s face and fail to provide compact low profiles for the viewer. Excessive weight of these systems causes head or neck strain and fatigue problems.
The operation of each of these night vision systems is similar. An objective lens collects light from a low illumination scene and focuses it onto a photocathode of an image intensifier tube. The photocathode converts this image into an electronic signal that is amplified and converted into an intensified image on a screen of the image tube. An eyepiece magnifies the screen image for viewing.
Several devices use a single objective to image a scene on a single image intensifier and split the output (ocular) side into two paths (for each eye) using mirrors or prisms. These devices are, for example, the US Army PVS-7 binocular night vision goggle and the Simrad GN1 Night Visions Goggle, disclosed in U.S. Pat. No. 5,712,726. The PVS-7 device does not have a low profile (extends 170 mm beyond the face of the viewer). The GN1 device lacks inter-pupilary adjustment.
Another device, known as the Filipovich/FJW compact see-through night vision goggles, is disclosed in U.S. Pat. No. 4,653,879. As disclosed, this device images a scene onto two separate image intensifier paths using conventional optical lenses and beam combining prisms to achieve a compact structure. The Filipovich device has a limited field of view and is heavy.
Yet another device, known as the Takahashi system, is disclosed in U.S. Pat. No. 5,699,194 and U.S. Pat. No. 5,701,202. A similar device, known as the Okuyama system, is disclosed in U.S. Pat. No. 5,706,136. Both devices use a compact, aspheric beam-combiner prism to directly superimpose an electronically generated scene on a directly viewed scene. The Takahashi and Okuyama systems have two major problems. While the optical performance is very good, the surfaces required to correct the aberrations can not be manufactured by normal polishing or single point diamond machining. Special very expensive molds with non-rotationally symmetric surfaces are required. Another problem is the input signal (image on the image intensifier) is oriented at an odd angle (38 degrees relative to the line-of-sight). An image intensifier and objective lens folded into such a system results in a high profile.
Another device, known as the Janeczko system, is disclosed in U.S. Pat. No. 6,088,165. The system uses a compact aspheric beam-combiner prism to superimpose an image from a folded image intensifier optical path and an image from a video input onto direct viewing by a viewer. While the Janeczko system has a lower profile (50 mm) than the PVS-7 system, the profile of the Janeczko system is still excessive.
A compact head-up display is disclosed by Upatneik in U.S. Pat. No. 4,711,512. The display uses two linear diffraction gratings on a planar waveguide for relaying an image from a CRT to a viewer""s eye. While the Upatneiks system may have good image quality, it requires a collimator with a physical diameter of about six times the diameter of the eye pupil (60 mm for 10 mm eye pupil). This is unacceptable from a weight and profile consideration. Collimators having a diameter of 60 mm and a focal length of 25 mm are also difficult to fabricate (F number=0.42). The efficiency of the display is also low because its diffractive gratings only cover an incident parallel ray bundle that subtends an angle of +/xe2x88x924 degrees before extinction.
To meet this and other needs, and in view of its purposes, the present invention provides an optical system for directing light from an image source to a viewer""s eye. The optical system includes at least one image source providing an image source light; a collimator for receiving the image source light and converting the received image source light into a collimated light projected along a first optical path; and a planar diffractive relay including opposing planar surfaces longitudinally oriented substantially along a second optical path, the opposing planar surfaces terminating into an input tilted surface and an output tilted surface. The collimated light is (a) projected along the first optical path, (b) redirected by the input tilted surface as propagated light traveling in the second optical path, and (c) the propagated light traveling in the second optical path is redirected by the output tilted surface into a third optical path directed toward the viewer""s eye.
In one embodiment, the input tilted surface and one surface of the opposing planar surfaces subtend an angle greater than half of a critical angle for producing substantially total internal reflection of the propagated light traveling in the second optical path. The input tilted surface includes a length dimension sufficiently large for receiving the collimated light projected from the collimator and redirecting the collimated light for the propagation in the second optical path.
In another embodiment, the output tilted surface and the one surface subtend an angle greater than half the critical angle for redirecting the propagated light into the third optical path.
It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.